Golden Eggsteroid #18 (OSIRIS-REx) is located on level 8-25 of Angry Birds Space Beak Impact. The OSIRIS-REx is hiding above the top planet directly above the slingshot. Fling the Lazer Bird up their to nab it. Once you do you’ll immediately be transported via a wormhole to the level (E-18).
If you need help, the video below shows the location of the Golden Eggsteroid a strategy for passing the level with 3 stars.